Plant responses to light competition: Does evolutionary history matter?

Plants can respond to light-competition cues with sets of plastic responses that provide either shade avoidance or tolerance and are suggested to match these strategies to the competition scenarios they experience. However, little is known about the effect of plants' evolutionary history on their ability to shift between these strategies. To fill this knowledge gap, we performed a common-garden experiment that examined shade avoidance and tolerance responses of the winter annual plant Hymenocarpos circinnatus to a variety of simulated light-competition scenarios, including different heights and densities of a surrounding vegetation. These responses were compared across plants originating from six populations along a climatic gradient: from a semi-arid region, where light competition is relatively weak and homogenous, to a mesic-Mediterranean region, where light competition is stronger and more heterogenous. In most of the studied traits, including those related to shade avoidance such as vertical elongation, or to shade tolerance such as photosynthetic efficiency, we found no evidence for differences among H. circinnatus populations in the extent of their responses to light competition. Interestingly, however, regardless of their evolutionary history, H. circinnatus were more responsive to cues of neighbour density rather than height in both shade tolerance and avoidance traits. Moreover, we found differences among populations in mean values of some of the studied traits across treatments, particularly in the onset of flowering and internode length, which are related to adaptations to the shorter and unpredictable growth season in the more arid sites. The little divergence we found in light-competition responses across populations along the climatic gradient might reflect low costs of plasticity in these traits in H. circinnatus. Moreover, our results indicate that procumbent plants such as H. circinnatus might respond more to lateral competition cues that indicate neighbour density than to vertical cues that indicate neighbour height, thus highlighting the need to incorporate the two types of cues when studying plastic responses of plants to competition. Read the free Plain Language Summary for this article on the Journal blog.